According to some, humanity's future lies in space. In addition to the proposals from nations like China, which have announced plans to establish an outpost on the Moon in the next decade, some private aerospace companies are looking to make regular trips to the Moon and beyond a reality.
Someday, this could lead to ventures like space tourism - where customers can book a trip to orbit, the Moon, and even Mars - and even the creation of commercial space stations, and lunar and Martian colonies.
For generations, human beings have fantasized about the day when people could live on the Moon or Mars. But with all the developments that have happened in the past decade or so, we are coming to the point where some of these ideas are starting to look more feasible.
This begs the question: how will human beings live in space over the long haul? Should we be planting our roots in the soil of other planets and altering them (and/or ourselves) to ensure our survival? Or should we look to creating orbiting habitats with microclimates and artificial gravity?
In terms of resources, time, effort and accommodation are space habitats the way to go? And from a strictly cost/benefit analysis, is it a better option than colonizing planets, moons, and other celestial objects?
During the Planetary Science Vision 2050 Workshop, which took place in February of 2017 at NASA's Headquarters in Washington DC, scientists from all over the world came together to share research and presentations about the future of humanity in space.
It was here that Valeriy Yakovlev - an astrophysicist and hydrogeologist from the Laboratory of Water Quality in Kharkiv, Ukraine, presented a paper titled, "Mars Terraforming – the Wrong Way".
Rather than colonizing and transforming the various bodies of the Solar System, he argued, humanity should instead construct space habitats. Addressing the idea of establishing a permanent colony on Mars, he claimed that:
"[A] radical obstacle to this is the unavailability of human beings to live in conditions of the reduced gravity of the Moon and Mars, being in their earthly bodies, at least in the next decades."
"If the path of space exploration is to create a colony on Mars and furthermore the subsequent attempts to terraform the planet, it will lead to the unjustified loss of time and money and increase the known risks of human civilization."
The reason for this, according to Yakolev, is because surface habitats and terraforming do not address the main challenges of colonizing space. His concern is that, rather than focusing on how to get to there or how we intend to go about creating the necessary infrastructure, the main challenge of living in space comes to down to the difficulty of having babies in space.
The hazards of living in space
Let's face it, there are no shortages of hazards when it comes to living in space. Besides the danger of living in a sealed, pressurized tin can that is the only thing between the occupants and the vacuum of space, there's also all manner of things that can kill you.
Micrometeoroids are one danger. These small particles of space debris can pose a threat to spacecraft operations in Earth's orbit. While tiny and weighing less than a gram (0.035 ounces), they can reach tremendous speeds and generate a significant force of impact.
The average velocity of micrometeoroids relative to a spacecraft in orbit is about 10 km/s (6.2 mi/s), which works out to 36,000 km/h (22,500 mph). While individual impacts are not likely to rupture a spacesuit or the hull of a spacecraft or space station, long term exposure can cause significant wear and tear.
Then there is the danger posed by radiation in space. Thanks to Earth's atmosphere and its protective magnetic field, human beings in developed nations such as the United States are exposed to an average background radiation of around 0.31 rem (3.1 mSv), with another 0.31 rem (3.1 mSv) per year from man-made sources.
However, beyond the protection of our atmosphere and magnetosphere, astronauts are exposed to much higher levels of solar radiation and galactic cosmic rays (GCR). There's also the elevated radiation that comes with solar particle events (SPE).
According to NASA studies, astronauts aboard the International Space Station (ISS) for six months are exposed to doses of ionizing radiation in the range of 50 to 2,000 mSv.
These and other studies have established an upper limit of 500 mSv per year for astronauts, which is the highest annual dose for which there was no observed increase in the rate at which cancer occurs in humans.
However, prolonged exposure dramatically increases the risk of acute radiation sickness, cancer, damage to the central nervous system, increased risk of degenerative disease, genetic damage, and even death.
Long-term effects of low gravity
On Earth, the force of gravity is equal to 9.8 meters per second per second (9.8 m/s²). This means that any object in freefall towards the surface accelerates at a rate of 9.8 meters (32 feet) for every second it is falling.
Long-term exposure to microgravity (which astronauts experience in orbit), or lower levels of gravity, can have detrimental effects on all living creatures that have evolved in "Earth-normal" gravity (or 1 g). Multiple studies have been conducted into this phenomenon, largely aboard the ISS.
This includes NASA's seminal Twins Study, where astronauts Scott and Mark Kelly were used for comparative analysis. While Scott Kelly acted as a test subject and spent a year aboard the ISS, Mark Kelly remained on Earth and acted as the control.
Multiple physicals were conducted on both astronauts after Scott Kelly returned to Earth. In addition to muscle and bone density loss, the studies showed that long-duration missions to space led to diminished organ function, eyesight, and even genetic changes. Re-adapting to Earth-normal gravity can also be arduous and painful for astronauts.
At present, it is entirely unknown whether or not medical advances can counteract these effects. It is also unknown whether or not rehabilitation strategies, such as those that involve centrifuges, will be effective over long periods of time.
This raises the question, why not simply establish habitats that are able to simulate Earth-normal gravity? Not only would inhabitants have no need for medical intervention to prevent physical degeneration, but they would also possibly be able to have children in space without additional worries about the effects of micro-gravity.
As for what kind of space habitats we could build, there are a number of options, all of which have been explored in science fiction and official studies.
History of the concept
Much like research into rocketry and space exploration, the idea of creating habitats in Earth orbit or space predates the Space Age, and goes back to the beginning of the 20th century.
It is also here that a great debt is owed to Konstantin Tsiolkovsky (1857 - 1935), one of the founding fathers of rocketry and aeronautics. In 1903, he published a study titled “Investigation of Outer Space Rocket Devices,” where he suggested using rotation to create artificial gravity in space.
In 1928, Sloven rocket engineer Herman Potočnik released his seminal book Das Problem der Befahrung des Weltraums der Raketen-Motor (The Problem of Space Travel - the Rocket Motor). Here, he suggested building a spinning, wheel-shaped station with a 30-meter (~100 foot) diameter that could be placed in geostationary orbit.
In 1929, Irish scientist John Desmond Bernal wrote "The World, the Flesh & the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul" in which he described a hollow spherical space habitat measuring 16 km (10 miles) in diameter, filled with air, and able to accommodate a population of 20,000 to 30,000 people.
In the 1950s, German-American rocket scientists Wernher von Braun and Willy Ley updated the idea as part of an article and spread for Colliers Magazine - titled "Man Will Conquer Space Soon!"
Von Braun and Ley envisioned a 3-deck, rotating wheel with a diameter of 76 meters (250 feet). This wheel would revolve at 3 rpm to provide artificial gravity (one-third of Earth's gravity), and act as a staging point for spacecraft headed to Mars.
In 1954, the German scientist Hermann Oberth described the use of massive, rotating cylinders for space travel in his book "People into space – New projects for rockets and space travel" (Menschen im Weltraum – Neue Projekte für Raketen- und Raumfahrt).
In 1975, NASA’s Ames Research Center and Stanford University jointly held the first annual NASA Summer Study. This ten-week program saw professors, technical directors and students come together to create a vision of how people might someday live in a large space colony.
The result of this was the Stanford Torus Space Settlement, a design for a wheel-like space station that would house 10,000 people and would rotate to provide the sensation of either Earth-normal or partial gravity.
In 1974, while teaching at Princeton University, physicist Gerard K. O'Neill proposed the concept of a rotating cylinder in outer space, which was detailed in a September 1974 article of Physics Today - titled "The Colonization of Space".
This idea was the result of a cooperative study where O'Neill's students were tasked with designing stations that would allow for the colonization of space by the 21st century, using materials extracted from the Moon and Near-Earth Asteroids (NEAs).
O'Neill expanded on this in his 1976 book, The High Frontier: Human Colonies in Space, emphasizing how these types of "islands in space" could be built using existing technology.
"We now have the technological ability to set up large communities in space," he wrote, "communities in which manufacturing, farming, and all other human activities could be carried out."
According to his description, this cylinder would consist of two counter-rotating cylinders measuring 8 km in diameter and 32 km long. This would provide artificial gravity while also canceling out any gyroscopic effects.
During the 1990s, several updated versions of these concepts were proposed, thanks in large part to the Space Settlement Contest launched by NASA and the NSS in 1994.
These included updated versions of O'Neill cylinders, Bernal Spheres, and wheel stations that would take advantage of the latest developments in technology and materials science.
In 2011, Mark Holderman and Edward Henderson - of NASA's Technology Applications Assessment Team (TAAT) - designed a concept for a rotating wheel space station. This was known as the Non-Atmospheric Universal Transport Intended for Lengthy United States Exploration (Nautilus-X).
The concept was originally proposed for long-duration missions (1 to 24 months) to limit the effects of microgravity on human health. More recently, the idea was explored as a possible sleep quarters module that would be integrated with the ISS.
This would make it possible to experiment with artificial gravity without destroying the usefulness of the ISS for experiments in microgravity. The research could also help refine concepts for spacecraft that are able to simulate gravity using a centrifuge.
In 2010, NASA began working to fulfill their vision for the future of human space exploration, now known as their "Moon to Mars" program. This program envisioned the development of a new generation of heavy launch vehicles, spacecraft, and space stations that would allow for human exploration beyond Earth.
A central part of the mission architecture is the Deep Space Gateway, an orbiting habitat which would be built in cis-lunar space. This habitat would facilitate future missions to the Moon for NASA, other space agencies, and commercial partners, while also serving as a staging point for missions to Mars.
In 2018, the propsed habitat was renamed the Lunar Orbital Platform-Gateway (LOP-G) - or just the Lunar Gateway. The proposed configuration calls for the creation of a modular station consisting of eight elements, contributed by NASA and international partners.
This station will serve as a stopover point where crews launched from Earth - using the Space Launch System (SLS) and the Orion space capsule - will be able to dock and resupply. Astronauts and commercial crews will be able to travel to the lunar surface will do so using a reusable lunar lander.
For missions headed to Mars, NASA plans on adding another spacecraft element - the Deep Space Transport. This reusable spacecraft will rely on Solar-Electric Propulsion (SEP) to make trips between the Lunar Gateway and another station in orbit around Mars.
This station is known as the Mars Base Camp, another modular station that will allow for astronauts to dock and resupply before going down to the Martian surface. This will be accommodated by the Mars Lander, another reusable spacecraft.
In January of 2016, the Keck Institute for Space Studies hosted a presentation at Caltech titled "Building the First Spaceport in Low-Earth Orbit". The lecture was presented by members of the Gateway Foundation, a non-profit organization dedicated to creating the world's first spaceport.
The Gateway’s design consists of two concentric inner rings fixed by four spokes to an outer ring. The inner rings make up the Lunar Gravity Area (LGA), where tourists will be able to dine and play in station’s rotation will simulate lunar gravity.
The outer ring (LGA Habitation ring) is where habitation modules are placed. The outer ring, known as the Mars Gravity Area (MGA), experiences faster rotation and provides an artificial gravitational environment similar to what people would be experienced on the surface of Mars.
The core of the station is where the Hub and Bay would reside. This is where the Gateway’s traffic control and operations would be coordinated from. The Hub will also have an observation lounge where guests can watch incoming shuttles.
The Gateway concept is one of many indications of the growing relevance and presence of the commercial aerospace industry in space. The Foundation also envisions that commercial launch providers like SpaceX will be invaluable in sending the Gateway's modules to orbit (using the Starship/Super Heavy launch system).
Benefits over surface colonies
Space colonies have their fair share of upsides and downsides. But compared to establishing colonies on planets, moons, and asteroids, there are a number of really favorable trade-offs.
For one, rotating space stations - whether they take the form of O'Neill Cylinders, Von Braun Wheels or Stanford Torii - can be spun up to the point that they can mimic Earth-normal gravity.
This would eliminate concerns about the long-term health effects of low-g, and allow colonists a better chance of having children without the need to rely on medical treatments or artificial methods.
Radiation protection could also be provided by ensuring that the stations' outer walls are reinforced with radiation-resistant material (like lead, depleted uranium, or wastewater). Additional shielding could possibly be provided by generating a magnetic field.
Space habitats could also allow for a great deal of flexibility when it comes to where to locate the colony. They could be built in orbit around Earth, the Moon, Mars, or possibly even other planets and major bodies in the Solar System.
They could also be positioned at any or all of the Lagrange Points throughout the Solar System. These are locations where the gravitational forces of a two-body system (like the Sun and the Earth) produce regions of enhanced equilibrium, where a spaceship can be 'parked'.
Challenges of making space habitats
Of course, no discussion about space habitats would be complete without mentioning the many challenges they present. Much like any effort to colonize beyond Earth, the most obvious one is cost.
To build a single habitat in orbit around Earth would require a considerable amount of building materials, fuel, and construction robots. As it stands, SpaceX's Falcon 9 and Falcon Heavy can deliver payloads to LEO at a rate of $2,719 and $1,410 per kg, respectively.
While the development of fully reusable vehicles - as well as smallsat launch services and single-stage-to-orbit (SSTO) rockets - has led to a significant reduction in launch costs, sending all the necessary materials and equipment into orbit would still be a monumental expenditure.
A possible solution would be to extract materials from NEAs or the Moon using robotic spacecraft and haulers. These could then be brought to Earth orbit to be processed into building materials and assembled using construction robots.
However, this would still require that a megatons worth of material and machinery be sent into space to build these robots and facilities. The costs become even more prohibitive the farther away these habitats are being built.
Way of the future?
However, this is another advantage of creating space habitats. While the initial investment to create them in orbit around Earth or in cis-lunar space would be immense, these habitats could serve as stepping stones to more distant locations.
Basically, having these habitats in place between Earth and the Moon would mean that spacecraft could be assembled in orbit using materials harvested from space. They would also be able to launch from these stations, rather than having to take off from Earth.
This would mean significant reductions in terms of the number of launches from Earth, not to mention the amount of fuel needed to mount deep-space missions.
From the Earth-Moon system, robotic spacecraft and crews could potentially be sent to Mars, the Asteroid Belt, and to the outer and inner Solar System to build additional habitats using materials harvested locally.
The more locations we have "colonized" with space habitats, the easier it will be to expand humanity's presence across the Solar System. However, it is unlikely that future generations would choose one option over the other.
In the end, it seems more realistic to assume that space habitats could facilitate the spread of human beings through space, which includes allowing for settlements on other planets. So in addition to "Martians" and the like, there would also be "Lagrangians" (or whatever name they go by).
- ESA - Space Gateway
- NSS - Space Settlements
- NASA - Gateway Memorandum
- NSS - O'Neill Cylinder Space Settlement
- The Gateway Foundation - The Gateway
- NASA - Space Settlements: A Design Study (1977)
- KISS - Building the First Spaceport in Low Earth Orbit
- NASA - Human Research Program: The Human Body in Space
- Lunar and Planetary Institute Mars Terraforming: The Wrong Way
- NASA - Deep Space Gateway to Open Opportunities for Distant Destinations
- NSS - "Colonies in Space", by T.A. Heppenheimer (1977)